The use of RF communications has vastly increased in recent years, in everything from cellular telephony, satellite communications and geographic position information, and data communications. As an example of data communications, wireless computer network communication systems have become less expensive to implement and maintain, and they are becoming more prevalent and more widely used to communicate data among nodes of a local area network (LAN).
As the supply of RF devices and protocols increases, so does the need to be able to test such equipment in controlled environments. However, current methods for testing RF communication equipment still present several problems. Testing methods typically range from simply setting up the test in an open air environment, to connecting the wireless equipment together via cables, to assembling test setups disposed within RF shielded rooms or enclosures.
When testing radio equipment, it is common to place a device under test (DUT) into a shielded enclosure so that it is isolated from other potentially interfering radio signals. To provide sufficient isolation of the DUT, the enclosure completely surrounds the DUT, and the enclosure cover or door is tightly closed during tests. RF ports are typically provided in the enclosure to allow intended radio signals to pass between the interior and exterior. Other than the ports, all RF signals must be blocked. This isolation requirement presents several problems.
One problem is that a DUT can generate a considerable amount of heat in normal operation. Even seemingly low power devices, when placed in an environment where the air is not free to circulate, can get very warm, endangering the electronics.
Another problem is that the DUT often requires an external power supply; hence electric power must also pass through the enclosure walls to reach the DUT. Since interfering RF signals can be carried on the power conductors, the power source must be filtered to block these signals.
A further problem is that many DUTs require some means of remote control. Additional specialized ports in the enclosure are needed for conveying control signals while blocking interfering RF signals.
Commercially-available enclosures do not provide enough flexibility or return on investment to the customer. The enclosures are either highly specialized in their electrical interfaces, or too general to be convenient. For instance, one may purchase an enclosure that has a filtered and isolated RS-232 serial interface built into it. This is a perfect match for DUTs with an RS-232 interface. However, if the customer would now like to use the enclosure to test a device that has, for instance, an Ethernet interface, the RS-232 interface is now a liability. The customer must now buy a new enclosure, one designed with Ethernet isolation in mind.
One alternative is to design an enclosure with multiple filtered or isolated electrical interfaces. However, this adds cost, for the customer may have to purchase a product that has numerous unused interfaces just to get the two or three they really need. There is an additional risk that sometime in the future this enclosure will become obsolete because a new interface, not supported by the existing general enclosure, becomes popular.
Another alternative is to provide a general means for the customer to convey any signal they need through the enclosure wall. This exists in the form of a circular filtering waveguide, essentially a metal tube of the proper diameter and length. This method is simple and general, but has several drawbacks. The first is a simple practical issue: the diameter and length of the waveguide depends on the highest frequency that needs to be blocked. This can limit the number and type of cables that can be passed through the waveguide.
The second issue is more serious. To truly isolate the interior from the exterior RF environment, the cables must be fiber optic. The waveguide is designed to block radiated signals, not conducted signals. Metallic cables act like antennas and pick up RF radiation in the environment and carry it through the waveguide, thereby defeating its purpose. For this reason, the waveguide technique, while seeming to be general solution, is very inconvenient and limiting, for the customer must further purchase the appropriate specialized optically-isolated interface mechanisms. This can be expensive, and can require further interface tradeoffs.
A similar situation exists with ventilation. The customer may need to purchase an enclosure with one or more fans, yet not need the fans for every test situation. The fans can be turned off, but being permanently mounted with the enclosure means the fans add cost to the enclosure, and can increase the space required for the enclosure. Additional enclosure functionality, such as a temperature monitor for the enclosure interior is in the same situation.
For these reasons, there is a need for a shielded enclosure that can provide great flexibility in the signals it can pass into out of the enclosure, but avoid the great expense of providing for all possible signal paths, along with the issue of changing interface requirements.